1. Field of the Invention
The present invention relates to a method for manufacturing a silicon wafer with an insulating intermediate layer.
2. Description of the Related Technology
Such a method for manufacturing a silicon wafer with an oxide layer as an insulating intermediate layer, hereinafter referred to as an SOI wafer is known from the publication EP 1 045 448 A. In the method described, which is also known as xe2x80x9cSMART CUTxe2x80x9d, a first silicon wafer is oxidized and then a hydrogen-rich layer is created by means of a hydrogen implantation in a depth defined by the implantation energy. The surface of the first wafer is then bonded to the surface of a second wafer, thus creating a buried insulating intermediate layer of oxide. A subsequent temperature step causes bubble formation in the hydrogen-rich layer which blows off the greater part of the first wafer. In order to achieve a surface or layer quality for the manufacture of integrated circuits, the new surface of the Sol wafer is freed by means of further process steps to the greatest possible extent from the contaminations and defects which the blowing off has caused in the layer near to the surface. The thicknesses of the silicon layer lying on the oxide layer which can be achieved with the method lie in the range of 1.0 xcexcm and are largely determined by the maximum penetration depth which the hydrogen ions can achieve during the implantation.
In other known methods, described for example in the publication W. P. Maszara, Electrochem. Soc. PV 90-6, 199-212 B (1990), SOI wafers are manufactured by reducing either the rear side of the first or the rear side of the second wafer to the desired layer thickness, which is usually more than a few xcexcm, by thinning, or cutting and thinning, after the oxidation and the bonding of the surface of the first wafer to the surface of the second wafer.
The disadvantage of the previous methods is that great expenditure is required to improve the surface and/or the layer quality of the silicon layer lying on the insulating oxide. In particular, the blowing off in the SMART-CUT method causes an exceptionally large amount of crystal damage in the surface of the wafer, which is reduced by means of subsequent process steps, such as for example chemical-mechanical polishing or reduction processes. As a result of the manner of manufacture with the other methods, the cleaning of the surface and the checking of the layer parameters, such as the evenness of the layer thickness, and the production of reproducible final thicknesses is also very time-consuming. Furthermore, buried layers or vertical gradients in the layer parameters, especially in the case of layer thicknesses greater than 2 xcexcm, cannot be manufactured with acceptable expenditure.
As the quality of the silicon layer is an important prerequisite for ensuring an acceptable yield in the manufacture of integrated circuits in the field of silicon technology, particularly in the field of manufacturing integrated circuits, it is an object of the development to provide methods which create a particularly high layer quality, i.e. evenness and freedom from defects, for silicon wafers with an insulating intermediate layer with acceptable expenditure. Furthermore, it is desirable to be able to easily adapt parameters, such as the conductivity, within the upper silicon layer even for thicker silicon layers.
The object of the present invention is to provide a method with which parameters of the surface layer of an SOI wafer can be easily adapted.
This object has been achieved according to the invention in a method for fabricating a manufactured multi-layered silicon wafer by bonding together a first silicon wafer and a second silicon wafer.
According to this, starting from a wafer that has an insulating intermediate layer, the essence of the invention lies in obtaining the silicon boundary surface of the silicon/oxide boundary layer of the first silicon layer as the new silicon surface. For this purpose, a silicon wafer with an insulating intermediate layer is created, starting from a first silicon wafer comprising a surface layer on top of an insulating intermediate layer arranged on a substrate and a second silicon wafer with a surface, by bonding the respective surfaces of the two wafers and, before bonding, at least one insulating layer is applied to the surface of at least one silicon wafer, and the substrate of the first silicon wafer is removed after bonding.
In so doing, it is advantageous to remove the substrate of the first silicon wafer selectively with regard to the insulating intermediate layer, as selective removal can be performed by robust and comparably inexpensive process steps, such as for example by a wet chemical process. In a subsequent process step, the previous boundary surface of the silicon/insulating layers, which was buried before the bonding, is made available as a new silicon surface by removing the insulating intermediate layer of the first wafer.
The advantage of the new method in comparison to the previous state-of-the-art is that the surface of the first silicon layer is made available with higher quality for further production processes, such as for example the manufacture of integrated circuits, without requiring after-treatment of the uppermost silicon layer. The method does not depend upon the thickness of the first layer of the first silicon wafer. Particularly in the case of thicker layers, the demands upon the quality of the surface of the first wafer, such as for example the evenness of the layer thickness, are substantially reduced as an insulating layer is deposited on the surface, and the previous surface is buried by bonding with another wafer, and becomes the boundary surface between the silicon/insulating layers. The demands upon the structure and quality of these layers are low as the substrate and the insulating layer of the first wafer are removed. Furthermore, with selective removal of the substrate, the intermediate layer serves only as a stop layer, so that the thickness of the intermediate layer may also be, for example, less than 0.1 xcexcm. The thermal load and stress from the oxidation are reduced to the greatest possible extent by the low demands upon the manufacturing process of the first wafer.
In a development of the method, it is advantageous if an oxide layer is created as an insulating layer on one of the two wafers as this only exerts low tensile and shear forces on the underlying substrate, in contrast to, for example, a nitride layer. In order to simplify the manufacturing process, it is also advantageous to produce the oxide layer exclusively on the surface of the first wafer, which in the case of thicker layers is preferably achieved by means of a deposition process.
In another development of the method, at least one characteristic parameter of the surface layer of the first wafer is changed before bonding the surfaces of the two wafers. In particular, the resistance of the first silicon layer of the first wafer can be changed by an implantation or precoating. Another possibility is to increase the thickness of the first layer, for example by means of an epitaxy process, and at the same time create a vertical gradient in the resistance of the first layer.